9 The kernel is the core of the DragonFly operating system. It is responsible for managing memory, enforcing security controls, networking, disk access, and much more. While more and more of DragonFly becomes dynamically configurable it is still occasionally necessary to reconfigure and recompile your kernel.

26 Traditionally, DragonFly has had what is called a ***monolithic*** kernel. This means that the kernel was one large program, supported a fixed list of devices, and if you wanted to change the kernel's behavior then you had to compile a new kernel, and then reboot your computer with the new kernel.

28 Today, DragonFly is rapidly moving to a model where much of the kernel's functionality is contained in modules which can be dynamically loaded and unloaded from the kernel as necessary. This allows the kernel to adapt to new hardware suddenly becoming available (such as PCMCIA cards in a laptop), or for new functionality to be brought into the kernel that was not necessary when the kernel was originally compiled. This is known as a modular kernel. Colloquially these are called KLDs.

30 Despite this, it is still necessary to carry out some static kernel configuration. In some cases this is because the functionality is so tied to the kernel that it can not be made dynamically loadable. In others it may simply be because no one has yet taken the time to write a dynamic loadable kernel module for that functionality yet.

32 Building a custom kernel is one of the most important rites of passage nearly every UNIX® user must endure. This process, while time consuming, will provide many benefits to your DragonFly system. Unlike the `GENERIC` kernel, which must support a wide range of hardware, a custom kernel only contains support for ***your*** PC's hardware. This has a number of benefits, such as:

38 * Less memory usage. A custom kernel often uses less memory than the `GENERIC` kernel, which is important because the kernel must always be present in real memory. For this reason, a custom kernel is especially useful on a system with a small amount of RAM.

50 First, let us take a quick tour of the kernel build directory. All directories mentioned will be relative to the main `/usr/src/sys` directory, which is also accessible through `/sys`. There are a number of subdirectories here representing different parts of the kernel, but the most important, for our purposes, is `config`, where you will edit your custom kernel configuration, and `compile`, which is the staging area where your kernel will be built. Notice the logical organization of the directory structure, with each supported device, file system, and option in its own subdirectory.

54 If there is ***not*** a `/usr/src/sys` directory on your system, then the kernel source has not been installed. One method to do this is via git. An alternative is to install the kernel source tree from the archive distributed on the DragonFly CD named `src-sys.tar.bz2`. This is especially useful when you do not have ready access to the internet. Use the Makefile in `/usr` to fetch the source or to unpack the archive. When installing kernel source only, use the alternate build procedure below.

74 Traditionally, this name is in all capital letters and, if you are maintaining multiple DragonFly machines with different hardware, it is a good idea to name it after your machine's hostname. We will call it `MYKERNEL` for the purpose of this example.

76 **Tip:** Storing your kernel config file directly under `/usr/src` can be a bad idea. If you are experiencing problems it can be tempting to just delete `/usr/src` and start again. Five seconds after you do that you realize that you have deleted your custom kernel config file. Do not edit `GENERIC` directly, as it may get overwritten the next time you [update your source tree](updating.html#UPDATING-SETUP), and your kernel modifications will be lost. You might want to keep your kernel config file elsewhere, and then create a symbolic link to the file in the `config` directory.

91 Now, edit `MYKERNEL` with your favorite text editor. If you are just starting out, the only editor available will probably be ***vi***, which is too complex to explain here, but is covered well in many books in the [bibliography](bibliography.html). However, DragonFly does offer an easier editor called ***ee*** which, if you are a beginner, should be your editor of choice. Feel free to change the comment lines at the top to reflect your configuration or the changes you have made to differentiate it from `GENERIC`.

93 If you have built a kernel under SunOS™ or some other BSD operating system, much of this file will be very familiar to you. If you are coming from some other operating system such as DOS, on the other hand, the `GENERIC` configuration file might seem overwhelming to you, so follow the descriptions in the [[Configuration File|handbook-kernelconfig-config]] section slowly and carefully.

99 **Note:** Be sure to always check the file `/usr/src/UPDATING`, before you perform any update steps, in the case you [sync your source tree](updating.html#UPDATING-SETUP) with the latest sources of the DragonFly project. In this file all important issues with updating DragonFly are typed out. `/usr/src/UPDATING` always fits your version of the DragonFly source, and is therefore more accurate for new information than the handbook.

122 If you have ***not*** upgraded your source tree in any way since the last time you successfully completed a `buildworld`-`installworld` cycle (you have not run `git pull` ), then it is safe to use the `quickworld` and `quickkernel`, `buildworld`, `buildkernel` sequence.

138 The installer copies the new kernel and modules to `/boot/kernel/`, the kernel being `/boot/kernel/kernel` and the modules being `/boot/kernel/*.ko`. The old kernel and modules are moved to `/boot/kernel.old/`. Now, shutdown the system and reboot to use your new kernel. In case something goes wrong, there are some [troubleshooting](kernelconfig-trouble.html) instructions at the end of this chapter. Be sure to read the section which explains how to recover in case your new kernel [does not boot](kernelconfig-trouble.html#KERNELCONFIG-NOBOOT).

141 **Note:** If you have added any new devices (such as sound cards), you may have to add some device nodes to your `/dev` directory before you can use them. For more information, take a look at device nodes section later on in this chapter.

147 <!-- XXX: do we really want to mention all these million config options? -->

148 The general format of a configuration file is quite simple. Each line contains a keyword and one or more arguments. For simplicity, most lines only contain one argument. Anything following a `#` is considered a comment and ignored. The following sections describe each keyword, generally in the order they are listed in `GENERIC`, although some related keywords have been grouped together in a single section (such as Networking) even though they are actually scattered throughout the `GENERIC` file. An exhaustive list of options and more detailed explanations of the device lines is present in the `LINT` configuration file, located in the same directory as `GENERIC`. If you are in doubt as to the purpose or necessity of a line, check first in `LINT`.

152 The following is an example `GENERIC` kernel configuration file with various additional comments where needed for clarity. This example should match your copy in `/usr/src/sys/config/GENERIC` fairly closely. For details of all the possible kernel options, see `/usr/src/sys/config/LINT`.

202 The above option specifies the type of CPU you have in your system. You may have multiple instances of the CPU line (i.e., you are not sure whether you should use `I586_CPU` or `I686_CPU`), however, for a custom kernel, it is best to specify only the CPU you have. If you are unsure of your CPU type, you can check the `/var/run/dmesg.boot` file to view your boot up messages.

212 This is the identification of the kernel. You should change this to whatever you named your kernel, i.e. `MYKERNEL` if you have followed the instructions of the previous examples. The value you put in the `ident` string will print when you boot up the kernel, so it is useful to give the new kernel a different name if you want to keep it separate from your usual kernel (i.e. you want to build an experimental kernel).

226 (Recommended) The system will auto-tune this setting for you if you explicitly set it to `0`[(1)](#FTN.AEN7414). If you want to manage it yourself you will want to set `maxusers` to at least 4, especially if you are using the X Window System or compiling software. The reason is that the most important table set by `maxusers` is the maximum number of processes, which is set to `20 + 16 * maxusers`, so if you set `maxusers` to 1, then you can only have 36 simultaneous processes, including the 18 or so that the system starts up at boot time, and the 15 or so you will probably create when you start the X Window System. Even a simple task like reading a manual page will start up nine processes to filter, decompress, and view it. Setting `maxusers` to 64 will allow you to have up to 1044 simultaneous processes, which should be enough for nearly all uses. If, however, you see the dreaded proc table full error when trying to start another program, or are running a server with a large number of simultaneous users, you can always increase the number and rebuild.

230 **Note:** `maxusers` does ***not*** limit the number of users which can log into your machine. It simply sets various table sizes to reasonable values considering the maximum number of users you will likely have on your system and how many processes each of them will be running. One keyword which ***does*** limit the number of simultaneous ***remote logins and X terminal windows*** is [kernelconfig-config.html#KERNELCONFIG-PTYS `pseudo-device pty 16`].

268 The normal build process of the DragonFly does not include debugging information when building the kernel and strips most symbols after the resulting kernel is linked, to save some space at the install location. If you are going to do tests of kernels in the DEVELOPMENT branch or develop changes of your own for the DragonFly kernel, you might want to uncomment this line. It will enable the use of the `-g` option which enables debugging information when passed to [gcc(1)](http://leaf.dragonflybsd.org/cgi/web-man?command#gcc&section1).

278 This line allows the kernel to simulate a math co-processor if your computer does not have one (386 or 486SX). If you have a 486DX, or a 386 or 486SX (with a separate 387 or 487 chip), or higher (Pentium®, Pentium II, etc.), you can comment this line out.

282 **Note:** The normal math co-processor emulation routines that come with DragonFly are ***not*** very accurate. If you do not have a math co-processor, and you need the best accuracy, it is recommended that you change this option to `GPL_MATH_EMULATE` to use the GNU math support, which is not included by default for licensing reasons.

292 Networking support. Leave this in, even if you do not plan to be connected to a network. Most programs require at least loopback networking (i.e., making network connections within your PC), so this is essentially mandatory.

324 This option includes functionality to speed up disk operations on large directories, at the expense of using additional memory. You would normally keep this for a large server, or interactive workstation, and remove it if you are using DragonFly on a smaller system where memory is at a premium and disk access speed is less important, such as a firewall.

334 This option enables Soft Updates in the kernel, this will help speed up write access on the disks. Even when this functionality is provided by the kernel, it must be turned on for specific disks. Review the output from [mount(8)](http://leaf.dragonflybsd.org/cgi/web-man?command#mount&section8) to see if Soft Updates is enabled for your system disks. If you do not see the `soft-updates` option then you will need to activate it using the [tunefs(8)](http://leaf.dragonflybsd.org/cgi/web-man?command=tunefs&section=8) (for existing filesystems) or [newfs(8)](http://leaf.dragonflybsd.org/cgi/web-man?command=newfs&section=8) (for new filesystems) commands.

346 This is the memory-mapped filesystem. This is basically a RAM disk for fast storage of temporary files, useful if you have a lot of swap space that you want to take advantage of. A perfect place to mount an MFS partition is on the `/tmp` directory, since many programs store temporary data here. To mount an MFS RAM disk on `/tmp`, add the following line to `/etc/fstab`:

378 The MS-DOS® Filesystem. Unless you plan to mount a DOS formatted hard drive partition at boot time, you can safely comment this out. It will be automatically loaded the first time you mount a DOS partition, as described above. Also, the excellent ***mtools*** software (in pkgsrc®) allows you to access DOS floppies without having to mount and unmount them (and does not require `MSDOSFS` at all).

390 The ISO 9660 Filesystem for CDROMs. Comment it out if you do not have a CDROM drive or only mount data CDs occasionally (since it will be dynamically loaded the first time you mount a data CD). Audio CDs do not need this Filesystem.

400 The process filesystem. This is a ***pretend*** filesystem mounted on `/proc` which allows programs like [ps(1)](http://leaf.dragonflybsd.org/cgi/web-man?command#ps&section1) to give you more information on what processes are running. ***

418 This causes the kernel to pause for 15 seconds before probing each SCSI device in your system. If you only have IDE hard drives, you can ignore this, otherwise you will probably want to lower this number, perhaps to five seconds (5000 ms), to speed up booting. Of course, if you do this, and DragonFly has trouble recognizing your SCSI devices, you will have to raise it back up.

428 Allow users to grab the console, which is useful for X users. For example, you can create a console ***xterm*** by typing `xterm -C`, which will display any [write(1)](http://leaf.dragonflybsd.org/cgi/web-man?command#write&section1), [talk(1)](http://leaf.dragonflybsd.org/cgi/web-man?command=talk&section=1), and any other messages you receive, as well as any console messages sent by the kernel.

468 This option provides for System V shared memory. The most common use of this is the XSHM extension in X, which many graphics-intensive programs will automatically take advantage of for extra speed. If you use X, you will definitely want to include this.

538 All PCs supported by DragonFly have one of these. Do not remove, even if you have no ISA slots. If you have an IBM PS/2 (Micro Channel Architecture), DragonFly provides some limited support at this time. For more information about the MCA support, see `/usr/src/sys/config/LINT`.

718 **Note:** The USB [umass(4)](http://leaf.dragonflybsd.org/cgi/web-man?command#umass&section4) driver (and a few other drivers) use the SCSI subsystem even though they are not real SCSI devices. Therefore make sure not to remove SCSI support, if any such drivers are included in the kernel configuration.

746 The keyboard controller (`atkbdc`) provides I/O services for the AT keyboard and PS/2 style pointing devices. This controller is required by the keyboard driver (`atkbd`) and the PS/2 pointing device driver (`psm`).

800 `sc0` is the default console driver, which resembles a SCO console. Since most full-screen programs access the console through a terminal database library like `termcap`, it should not matter whether you use this or `vt0`, the `VT220` compatible console driver. When you log in, set your `TERM` variable to `scoansi` if full-screen programs have trouble running under this console.

820 This is a VT220-compatible console driver, backward compatible to VT100/102. It works well on some laptops which have hardware incompatibilities with `sc0`. Also set your `TERM` variable to `vt100` or `vt220` when you log in. This driver might also prove useful when connecting to a large number of different machines over the network, where `termcap` or `terminfo` entries for the `sc0` device are often not available -- `vt100` should be available on virtually any platform.

870 **Note:** If you have an internal modem on COM4 and a serial port at COM2, you will have to change the IRQ of the modem to 2 (for obscure technical reasons, IRQ2 # IRQ 9) in order to access it from DragonFly. If you have a multiport serial card, check the manual page for [sio(4)](http://leaf.dragonflybsd.org/cgi/web-man?commandsio&section=4) for more information on the proper values for these lines. Some video cards (notably those based on S3 chips) use IO addresses in the form of `0x*2e8`, and since many cheap serial cards do not fully decode the 16-bit IO address space, they clash with these cards making the COM4 port practically unavailable.

972 MII bus support is required for some PCI 10/100 Ethernet NICs, namely those which use MII-compliant transceivers or implement transceiver control interfaces that operate like an MII. Adding `device miibus` to the kernel config pulls in support for the generic miibus API and all of the PHY drivers, including a generic one for PHYs that are not specifically handled by an individual driver.

1072 `sl` is for SLIP support. This has been almost entirely supplanted by PPP, which is easier to set up, better suited for modem-to-modem connection, and more powerful. The ***number*** after `sl` specifies how many simultaneous SLIP sessions to support.

1082 This is for kernel PPP support for dial-up connections. There is also a version of PPP implemented as a userland application that uses `tun` and offers more flexibility and features such as demand dialing. The ***number*** after `ppp` specifies how many simultaneous PPP connections to support. .

1092 This is used by the userland PPP software. A ***number*** after `tun` specifies the number of simultaneous PPP sessions to support. See the [userppp.html PPP] section of this book for more information.

1102 This is a ***pseudo-terminal*** or simulated login port. It is used by incoming `telnet` and `rlogin` sessions, ***xterm***, and some other applications such as ***Emacs***. The ***number*** after `pty` indicates the number of `pty`s to create. If you need more than the default of 16 simultaneous ***xterm*** windows and/or remote logins, be sure to increase this number accordingly, up to a maximum of 256. ***

1144 This is the Berkeley Packet Filter. This pseudo-device allows network interfaces to be placed in promiscuous mode, capturing every packet on a broadcast network (e.g., an Ethernet). These packets can be captured to disk and or examined with the [tcpdump(1)](http://leaf.dragonflybsd.org/cgi/web-man?command#tcpdump&section1) program.

1148 **Note:** The [bpf(4)](http://leaf.dragonflybsd.org/cgi/web-man?command#bpf&section4) device is also used by [dhclient(8)](http://leaf.dragonflybsd.org/cgi/web-man?command=dhclient&section=8) to obtain the IP address of the default router (gateway) and so on. If you use DHCP, leave this uncommented.

1208 Almost every device in the kernel has a corresponding node entry in the `/dev` directory. These nodes look like regular files, but are actually special entries into the kernel which programs use to access the device.

1217 **Note:** If you are having trouble building a kernel, make sure to keep a `GENERIC`, or some other kernel that is known to work on hand as a different name that will not get erased on the next build. You cannot rely on `kernel.old` because when installing a new kernel, `kernel.old` is overwritten with the last installed kernel which may be non-functional. Also, as soon as possible, move the working kernel to the proper `kernel` location or commands such as [ps(1)](http://leaf.dragonflybsd.org/cgi/web-man?command#ps&section1) will not work properly. The proper command to ***unlock*** the kernel file that `make` installs (in order to move another kernel back permanently) is:

1223 If you find you cannot do this, you are probably running at a [securelevel(8)](http://leaf.dragonflybsd.org/cgi/web-man?command#securelevel&section8) greater than zero. Edit `kern_securelevel` in `/etc/rc.conf` and set it to `-1`, then reboot. You can change it back to its previous setting when you are happy with your new kernel.

1238 * `config` fails: If the [config(8)](http://leaf.dragonflybsd.org/cgi/web-man?command#config&section8) command fails when you give it your kernel description, you have probably made a simple error somewhere. Fortunately, [config(8)](http://leaf.dragonflybsd.org/cgi/web-man?command=config&section=8) will print the line number that it had trouble with, so you can quickly skip to it with ***vi***. For example, if you see `config: line 17: syntax error`. You can skip to the problem in ***vi*** by typing `17G` in command mode. Make sure the keyword is typed correctly, by comparing it to the `GENERIC` kernel or another reference.

1243 * `make` fails: If the `make` command fails, it usually signals an error in your kernel description, but not severe enough for [config(8)](http://leaf.dragonflybsd.org/cgi/web-man?command#config&section8) to catch it. Again, look over your configuration, and if you still cannot resolve the problem, send mail to the [DragonFly Bugs mailing list](http://leaf.dragonflybsd.org/mailarchive/) with your kernel configuration, and it should be diagnosed very quickly.

1248 * Installing the new kernel fails: If the kernel compiled fine, but failed to install (the `make install` or `make installkernel` command failed), the first thing to check is if your system is running at securelevel 1 or higher (see [init(8)](http://leaf.dragonflybsd.org/cgi/web-man?command#init&section8)). The kernel installation tries to remove the immutable flag from your kernel and set the immutable flag on the new one. Since securelevel 1 or higher prevents unsetting the immutable flag for any files on the system, the kernel installation needs to be performed at securelevel 0 or lower.

1253 * The kernel does not boot: If your new kernel does not boot, or fails to recognize your devices, do not panic! Fortunately, DragonFly has an excellent mechanism for recovering from incompatible kernels. Simply choose the kernel you want to boot from at the DragonFly boot loader. You can access this when the system counts down from 10. Hit any key except for the **Enter** key, type `unload` and then type `boot ***kernel.old***`, or the filename of any other kernel that will boot properly. When reconfiguring a kernel, it is always a good idea to keep a kernel that is known to work on hand. After booting with a good kernel you can check over your configuration file and try to build it again. One helpful resource is the `/var/log/messages` file which records, among other things, all of the kernel messages from every successful boot. Also, the [dmesg(8)](http://leaf.dragonflybsd.org/cgi/web-man?command#dmesg&section8) command will print the kernel messages from the current boot.

1257 * The kernel works, but [ps(1)](http://leaf.dragonflybsd.org/cgi/web-man?command#ps&section1) does not work any more: If you have installed a different version of the kernel from the one that the system utilities have been built with, many system-status commands like [ps(1)](http://leaf.dragonflybsd.org/cgi/web-man?command=ps&section=1) and [vmstat(8)](http://leaf.dragonflybsd.org/cgi/web-man?command=vmstat&section=8) will not work any more. You must recompile the `libkvm` library as well as these utilities. This is one reason it is not normally a good idea to use a different version of the kernel from the rest of the operating system.